Biocompatible magnetic nanosensors have been designed to detect molecular interactions in biological media. Upon target binding, these nanosensors cause changes in the spin-spin relaxation times of neighboring water molecules (or any solvent molecule with free hydrogens) of a sample, which can be detected by classical magnetic resonance (NMR/MRI) techniques. Thus, by using these nanosensors in a liquid sample, it is possible to detect the presence of an analyte at very low concentration—for example, small molecules, specific DNA, RNA, proteins, carbohydrates, organisms, and pathogens (e.g. viruses)—with sensitivity in the low femtomole range (from about 0.5 to about 30 fmol).
In general, magnetic nanosensors are superparamagnetic nanoparticles that bind or otherwise link to their intended molecular target to form clusters (aggregates) or nanoassemblies. It is thought that when superparamagnetic nanoparticles assemble into clusters and the effective cross sectional area becomes larger, the nanoassembly becomes more efficient at dephasing the spins of surrounding water (or other solvent) protons, leading to an enhancement of the measured relaxation rates (1/T2). Additionally, nanoassembly formation can be designed to be reversible (e.g., by temperature shift, chemical cleavage, pH shift, etc.) so that “forward” or “reverse” assays can be developed for detection of specific analytes. Forward (clustering) and reverse (declustering) types of assays can be used to detect a wide variety of biologically relevant materials. Furthermore, the spin-lattice relaxation time (T1) is considered independent of nanoparticle assembly formation and can be used to measure concentration in both nano-assembled and dispersed states within the same solution.
Examples of magnetic nanosensors are described in Perez et al., “Use of Magnetic Nanoparticles as Nanosensors to Probe for Molecular Interactions,” ChemBioChem, 2004, 5, 261-264, and in U.S. Patent Application Publication No. US2003/0092029 (Josephson et al.), the texts of which are incorporated by reference herein, in their entirety. Examples of magnetic nanosensors include monocrystalline iron oxide nanoparticles from about 3 to about 5 nm in diameter surrounded with a dextran coating approximately 10 nm thick such that the average resulting particle size is from about 25 to about 30 nm.
More stably coated and amino-functionalized nanosensors can be prepared, for example, by cross-linking the dextran coating of the metal oxide particle core with epichlorohydrin, then treating with ammonia to provide functional amino groups. Aminated cross-linked iron oxide nanoparticles (amino-CLIO) have been made with 40 amino groups per particle, with an average particle size from about 40 to about 50 nm. These particles can withstand harsh treatment, such as incubation at 120° C. for 30 minutes, without a change in size or loss of their dextran coat. Amino groups in amino-CLIO can react by N-hydroxysuccinimide (NHS) based bifunctional cross-linking, allowing attachment of a range of sulfhydryl-bearing biomolecules. This gives rise to biomolecule-nanoparticle conjugates with unique biological properties. In addition to their use as sensors, the resultant superparamagnetic nanoparticles are valuable for imaging specific molecular targets, and as reagents for cell labeling and tracking.
Current diagnostic systems involve, for example, microarray technology, polymerase chain reaction (PCR), in situ hybridization, antibody-based immunoassays (e.g. enzyme-linked immunosorbant assays), chemiluminescence, nephelometry, and/or photometry. These systems cannot perform the diversity of assays at high sensitivity that is possible with an NMR-based nanosensor system.
Various non-NMR-based point of care bio-assays have been developed, such as portable blood glucose meters that operate using test strips impregnated with glucose oxidase. However, these systems are generally not as reliable as central hospital assays because they lack the sensitivity, calibration, and maintenance that a laboratory setting provides. These portable systems also lack the sensitivity that is possible with NMR-based nanosensor systems, and they cannot be easily adapted for multiple analyte detection.
The above-cited Josephson et al. and Perez et al. documents describe application of classical NMR relaxation methods with nanosensors using off-the-shelf relaxometers and MRI units. However, these units require large NMR RF coils and large magnets and are bulky, expensive, and are not tailored for use with magnetic nanosensors.
There is a need for a less expensive, commercially-realizable NMR-based analyte detection device suitable for use with magnetic nanosensors.